A semiconductor wafer typically presents a patterned structure, wherein the pattern is in the form of multiple-layer stacks. Various steps in the manufacture of semiconductor devices require measurements of thickness and/or other characteristics of each layer in the semiconductor wafer. NovaScan machines commercially available from Nova Measuring Instruments Ltd. utilize spectrophotometric thickness measurements applied to predetermined sites on the wafer, each containing a known layer stack described by a predefined optical model based on optical properties of all the stack layers.
It is especially important to determine the optical properties of each layer (film) of the actual stack resulting from the manufacturing steps and before performing further measurements such as measurement of the uppermost layer thickness, etc. Unfortunately, in cases when measurements are performed on the entire stack, the accurate determination of the properties of each separate layer is difficult to implement, and in some cases, even impossible, because of too many independent parameters to be involved, and thus causing uncertainty in data interpretation. In order to exclude such uncertainty, usually each unkown layer is separated by its deposition on a known substrate without other layers of the multi-layer stack and measured independently. However, the properties of the individually deposited layer may be different from those of the similar layer that passes all processing steps during the manufacture of a required multi-level stack.